Fluke 91 Series II User Manual page 159

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The real frequency (Hz) measurement is an automatic
measurement that occurs on the first full period contained
between the cursors. If the cursors are too close together
for a full period to be recognized, the test tool is unable to
make a measurement and you see an answer of "---". To
correct this situation move the right cursor more to the
right. This frequency function makes simple measurements
of sub-frequencies possible. Just move the left cursor to
the start of the waveform detail you want to measure and
read the frequency.
The rise time measurement also occurs on the first edge
following the left cursor. You must set the left cursor at the
waveform low level and the right cursor at the waveform
high level. These two levels are used as the 0% and 100%
references for this measurement. Rise time is defined as
the time taken for the waveform to go from 10% to 90% of
these two reference levels.
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To see the markers:
1. Press to open the MORE SCOPE window.
2. Press twice for page 3 of the MORE SCOPE
window.
3. Press to open the SET MARKERS list box.
4. Use to highlight "rise time", and press
to select.
5. Press to close the window.
You now have two 'X' markers on the leading edge of the
waveform, which indicate where the 10% and 90% levels
are. If you move the left cursor to the right, you see these
markers move too.
To make a fall time measurement, you must set the left
cursor at the high level and the right cursor at the low level.
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